Species, Units of Evolution, and Secondary Substance A thesis presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Master of Arts Daniel J. Molter June 2011 © 2011 Daniel J. Molter. All Rights Reserved. 2 This thesis titled Species, Units of Evolution, and Secondary Substance by DANIEL J. MOLTER has been approved for the Department of Philosophy and the College of Arts and Sciences by Arthur Zucker Associate Professor of Philosophy Benjamin M. Ogles Dean, College of Arts and Sciences 3 ABSTRACT MOLTER, DANIEL J., M.A., June 2011, Philosophy Species, Units of Evolution, and Secondary Substance Director ofThesis: Arthur Zucker Species are classes of organisms on the traditional view, but David Hull argues that species as units of evolution are better understood as individuals composed of organisms as their parts. Phillip Kitcher counters that evolutionary species are better understood as sets. Following Elliot Sober, I argue that constituent definition prohibits a set-theoretic interpretation of species. Following John Dupré, I argue that species and units of evolution are ontologically distinct entities that require different names. “Species” is the proper name of the species category, a class containing many intensionally-defined classes of organisms. The units of evolution that Hull describes are spatiotemporally-individuated physical objects. Following the principle of priority in biological nomenclature, Hull's transgenerational biological individual cannot be called "Species", because that name has a prior valid use. I argue that Hull's species-as- individual has a prior valid name which can be found in Aristotle's Categories. Organisms are members of species, but they are parts of Secondary Substances. Approved: _____________________________________________________________ Arthur Zucker Associate Professor of Philosophy 4 TABLE OF CONTENTS Page Abstract ............................................................................................................................... 3 Chapter 1: Historical Introduction ...................................................................................... 5 Chapter 2: Species as Individuals: A Critique of Hull ........................................................ 10 Chapter 3: Why Species are not Sets ................................................................................ 46 Chapter 4: Species as Units of Classification .................................................................... 54 Chapter 5: Units of Evolution as Substances .................................................................... 63 Chapter 6: Postscript on Species and Units of Evolution.................................................. 69 References ........................................................................................................................ 70 5 CHAPTER 1: HISTORICAL INTRODUCTION Until Darwin published The Origin of Species in 1859, scientific consensus held that species were unchanging natural kinds. Each organism was thought to share a common form characteristic of its species. An individual dog, for example, was thought to be an instantiation of the essence of dog in a particular quantity of matter. Being an instance of an essential form was taken as necessary and sufficient for membership in a species. The notion of species as immutable natural kinds has its origins in ancient Greek philosophy. Plato held that plants and animals on earth derived their being from participation in perfect eternal forms. All pine trees, for example, were thought to be reflections of the perfect form of pine tree that exists in an otherworldly immaterial realm of forms. On the Platonic view, the species 'pine tree' is the collection of all individuals that derive their essence from participation in the perfect unchanging form of pine tree. Aristotle also held that members of a species share a common essential form, but he rejected the Platonic notion of an immaterial realm of forms. The world itself is eternal on the Aristotelian view and the forms of plants and animals exist only in individual organisms. When plants and animals reproduce, their forms are copied in new matter, such that when an organism dies, its form does not perish but continues on, re- instantiated in the matter of its progeny. 6 Linnaeus inherited the Aristotelian notion of species as forms, but he modified the view to make it compatible with the Genesis story of creation. “We reckon the number of SPECIES as the number of different forms that were created in the beginning” (Linnaeus 1751, 157). For Linnaeus, species were collections of animals and plants descended without modification from a single pair of created ancestors. Linnaeus’s system of classification is a nested hierarchy of classes composed of five ranks: classes, orders, genera, species, and varieties (Linnaeus 1751, 155). The five ranks are based on the five categories of scholastic logical division: genus summum, genus intermedium, genus proximum, species, and individuum (Stafleu 1971, 28). Each rank in the hierarchy is related to the rank above it and to the rank below it by the genus-species relation. “The genus of the genera is the order, and the genus of the orders is the class” (Linnaeus 1751, 204). In this system of logical division, a genus is a general class of objects that possess common properties, and a species is a sub-division of the genus. Members of a species have all the properties required for membership in the genus, but they possess additional properties that differentiate them from other species in the genus. The genus-species relation is one of general sameness and specific difference. Though their views on species differ in important ways, Plato, Aristotle, and Linnaeus all held an essentialist species concept. That is, they all thought of species as collections of organisms sharing a common immutable form. On this view, the form of a thing is its essence, and it is the essence that determines what a thing is. Each thing can 7 have but a single essence, hence, every individual organism is a member of one and only one species. The goal of taxonomy under an essentialist species concept is to describe and catalog living things according to their essential forms (Popper 1950, Hull 1965, Mayr 1969, Lehmann 1971; in Stafleu 1971, 25). After Darwin's publication of The Origin of Species, the essentialist species concept became untenable. If one species can evolve into another, then the forms of living things do not remain constant from generation to generation. All essentialist accounts of species are refuted by the fact that gradual evolutionary processes cause the diversity of forms in nature. Essentialism was dead as a biological theory, but the concept of species did not fall with it. There are obviously specific kinds of plants and animals in nature, and that fact had to be explained in light of essentialism’s replacement by the theory of evolution. In 1942, Ernst Mayr gave an account that he called the biological species concept. "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups." Mayr’s species concept avoids morphological essentialism by defining species in terms of relations between the members of a species. The forms of organisms in a species change over time in accordance with evolution, but the mating compatibility relation remains unchanged. Groups of organisms meet the conditions for membership in a biological species by maintaining reproductive compatibility in each generation of an evolving lineage. 8 Mayr's formulation captured the popular notion of what species are within the animal kingdom, but it had some weaknesses. It said nothing about species of asexual organisms that do not breed, and it was of little use for determining species of plants, which often form hybrids. Attempts to fill in the gaps left by the biological species concept resulted in the publication of more species concepts. By the late 20th century, over twenty different species concepts had appeared in journals of philosophy and biology (Mayden 1997). Widespread acceptance of evolutionary theory ended the reign of essentialism as the dominant scientific theory, and it spawned a plethora of other a species concepts. On the traditional view, all species concepts reduce to conditions for class membership. Species concepts fall into three broad categories: phenetic, cohesive, and monophyletic (Hey 2006). The three categories of species concepts are differentiated by the kinds of properties used as criteria for class membership. Phenetic species concepts compare the physical properties of organisms, cohesive species concepts consider relational properties, that is, how organisms relate to other organisms, and monophyletic species concepts rely on inferred properties, that is, organisms are segregated into species based on inferences about the history of their evolution. In 1976, David Hull challenged the traditional notion of species as classes, claiming instead that species are individuals. In this master’s thesis I defend the traditional notion of species as classes. In the next chapter I examine Hull’s arguments for conceiving of species as individuals, and I argue that Hull has presented a mix of 9 good and bad arguments. In the third chapter I consider Philip Kitcher’s claim that species are sets rather than individuals, and I argue that species cannot be sets. In chapter four I outline John